The dihydric phenols have achieved significant success in their commercial applications. Dihydric phenols are useful in the commercial manufacture of various polymers including the polyarylates, polyamides, epoxies, polyetherimides, polysulfones and the polycarbonates. Significant attention has been directed to the commercial preparations of the dihydric phenols. For many years it has been well known that the acid catalyzed reaction of phenol with specific aldehyde or ketone could prepare the 4,4'-dihydric phenol with specific groups derived from the aldehyde or the ketone connecting the two phenolic rings. In particular when phenol is reacted with acetone, the dihydric phenol 4,4'(hydroxyphenyl)propane-2, hereafter known as bisphenol-A is formed. This has particular utility in polycarbonates, polyarylates and copolyestercarbonates as well as epoxies. In order to make certain polymers, in particular the polycarbonates, the bisphenol-A must be particularly pure, for example, as measured by color. Additionally, the process should be particularly efficient since the dihydric phenol costs contribute substantially to the cost of the final polymer. Therefore much attention has been directed to the recovery of bisphenol-A after preparation. Not only is recovery from the major stream containing primarily bisphenol-A important, but because of the economics involved, various side streams or "purge streams" also containing significant quantities of bisphenol-A should also be investigated for improved recovery techniques.
Various catalytic systems for acid catalysis of the reaction between phenol and bisphenol-A have been investigated and used commercially. At one time the hydrochloric acid catalyzed process was used in a significant number of commercial facilities. However the corrosion caused by the hydrochloric acid on standard metallic reactors and pre and post reaction equipment left much to be desired as far as replacement economics was concerned. Recently, substantial attention has been placed on using an ion exchange resin catalyst system since it does not have a significant acid corrosion problem. However it has recently been discovered in our equipment that the usual processing techniques for recovery of bisphenol-A from recovery streams having relatively small amounts of bisphenol-A, about 6 to 15 weight percent of the process stream, after preparation with the ion exchange catalyst cannot be practiced in the same manner as when using the hydrochloric acid catalyst system. Substantial quantities of bisphenol-A which could be isolated from streams having a major quantity of phenol after reaction with an HCl catalyzed system could no longer be recovered when using an ion exchange system. Additionally the quality of the bisphenol-A which could be recovered as well as the quality of other materials in these "purge streams" was sufficiently lessened as measured by the color of the materials. Color is a very important property of the final polymers which are prepared from the bisphenol-A as well as the bisphenol-A itself. For example, bisphenol-A polycarbonate is known to be clear and colorless.
It has now been discovered that bisphenol-A can be successfully recovered in substantial quantities from the purge streams of an ion exchange catalyzed reaction of a phenol with a ketone, particularly phenol per se with acetone, by utilizing a relatively simple treatment with a specific basic system. Not only is the dihydric phenol recoverable from the purge streams, but the color of the dihydric phenol and the recyclable components is substantially improved.